Patent Document 1 indicated below discloses a polyethylene spout stopper to be applied to a paper container having at least a synthetic resin film laminated to the inner surface thereof. Such a spout stopper comprises a protruding tubular wall allowed to protrude through a spout hole formed in the container, and an annular flange wall extending out radially outwardly from the outer periphery of a lower end part of the protruding tubular wall. The upper surface of an outer peripheral edge part of the annular flange wall is welded to the inner surface of the container by an ultrasonic welding method. An annular score is formed in an inner peripheral edge part of the annular flange wall. For the purpose of so-called sorted trash collection for recovery, such an annular score is broken when the container is to be discarded after the contents of the container are consumed, so that most of the spout stopper is separated from the container. In an intermediate part in the radial direction of the annular flange, a corrugated portion or stepped portion is further formed for preventing the occurrence of a defect, such as a pinhole, in the annular score by an ultrasonic impact during the execution of welding by the ultrasonic welding method.